Chemical and physical reactions cannot occur until individual molecules of the reagents are brought together, and the required physical interaction is greatly facilitated as the reagents are more and more intimately mixed together. Bulk stirring is only able to cause reagent molecules to contact one another after sufficient time has elapsed to provide the necessary uniformity of inter-dispersion of the reagents, and only natural molecular diffusion can accomplish the required one on one contact, which is a very slow process. These encounters can be helped to occur by the use of reactors of small enough scale within which molecular diffusion becomes significant when at least one of its dimensions is very small. The role of the reactor, and the mixing and mass transfer equipment associated with it, is to create sufficiently small scale fluid structures or eddies in order to generate and improve the uniformity of mixing, mass transfer and molecular interdiffusion. Many different types of reactors have been proposed, and are in commercial use, and may be classified broadly as being either of natural diffusion or forced diffusion type.
There is ongoing interest in what is referred to as process intensification technology, fuelled primarily by the need to provide industrial processes that are more efficient and economical than those employed to date. Such technology is applied to any physical and/or chemical process involving heat and/or mass transfer and/or physical and/or chemical reaction, the latter term including both chemical composition and decomposition.
One type of natural diffusion type reactor that has been proposed comprises a so-called micro-mixer manufactured using methods borrowed from the electronics industry. For example, such a reactor may consist of a series of very small channels engraved or etched into a silicon wafer surface, through which the reaction components are passed together in laminar flow mode; the channels can be as small as 10 micrometers in transverse dimension. Despite the improved mass transfer obtainable, many reactions are relatively slow because they are still natural diffusion controlled, and therefore their rate depends on slow, unforced, molecular inter-diffusion.
The forced diffusion type of reactor generally involves producing on, and/or introducing to, a moving surface a thin film or its equivalent of each of the reaction components, so that interaction between them is greatly facilitated. It is also found that such interactions are possible under conditions of temperature and/or pressure that can be relatively closely controlled, especially as compared with bulk stirring. When a process component has the form of a gas, vapour, or plasma, it may be introduced to the surface in a form which is equivalent to a thin film, for example by bathing the surface in the component, or as a flow of the required thin dimension.
A more specific type of forced diffusion reactor is what is now generally known as a spinning tube in tube reactor which, as its name implies, usually comprises a first cylindrical tube, usually the rotor, mounted within a second cylindrical tube of larger diameter so as to be rotatable about a common longitudinal axis with the operative exterior surface of the rotor tube spaced radially a very small distance (e.g. 300 micrometers or less) from the cooperating operative interior surface of the stator tube. The tubes usually are of uniform diameters along their lengths and the constant radius annular space between the two cooperating surfaces constitutes a reaction passage, consequently usually of uniform radial spacing along its length, through which the reactants pass while subjected to intense shear produced by their movement through the narrow passage and by the relative rotation between the operative tube surfaces.
A major problem with such spinning tube in tube reactors and the reactions that they involve are providing adequate uniformity throughout the length of the reaction passage of the radial spacing between the operative surfaces. In the absence of such uniformity there is the possibility that different parts of the reaction passage will produce reactions operating at different rates with the possibility of producing widely different end products.